Aluminum alloy sheet for structural components

ABSTRACT

Disclosed is a 7xxx-series aluminum alloy sheet that has a specific chemical composition and is produced by a common procedure. The sheet maintains high strength via a good balance between Zn and Mg, while the Zn content is controlled. The average compositional ratio of Zn to Mg in grain-boundary precipitates is controlled in the sheet that is naturally aged after production; and the average compositional ratio of Zn to Mg in intragranular precipitates is controlled in the sheet that is further artificially aged after the natural aging. Thus, the aluminum alloy sheet is allowed to have formability, corrosion resistance, and such high strength as to be demanded of structural components.

TECHNICAL FIELD

The present invention relates to a high-strength aluminum alloy sheet for structural components, where the aluminum alloy sheet offers better workability and still has excellent corrosion resistance. As used herein the term “aluminum alloy sheet” refers to a rolled sheet that is obtained after subjecting a sheet produced via rolling to solution treatment and quenching and subsequently to natural aging at room temperature for 2 weeks or longer and is one before forming into a structural component and before artificial aging. Also as used herein the term “microstructure” of the sheet after natural aging at room temperature refers to the microstructure of the sheet after subjected to natural aging at room temperature for 2 weeks or longer subsequent to solution treatment and quenching.

BACKGROUND ART

Social requirements for reduction in automobile body weight have increased more and more in consideration typically of global environment. To meet these requirements, aluminum alloy materials are applied typically to, out of the automobile body, panels and reinforcing materials, so as to replace part of steel materials such as steel sheets. The panels are exemplified by outer panels and inner panels typically of hoods, doors, and rods. The reinforcing materials are exemplified by bumper reinforcements (bumper RIFs) and door beams.

However, further reduction in automobile body weight requires the application of aluminum alloy materials also to automobile structural components, which are, of automobile components, particularly contribute to weight reduction and are exemplified by frames and pillars. These automobile structural components require higher strength, such as a 0.2% yield strength of 350 MPa or more, as compared with the automobile panels. JIS or AA 6xxx-series aluminum alloy sheets are used in the automobile panels and offer excellent formability, strength, and corrosion resistance and have a low-alloy chemical composition to offer excellent recyclability. The 6xxx-series aluminum alloy sheets, however, are far from the high strength as mentioned above, even when the chemical composition and temper (heat treatments) are controlled. The temper is exemplified by solution treatment and quenching, and further artificial aging.

Accordingly, the high-strength automobile structural components require the use of JIS or AA 7xxx-series aluminum alloy sheets, which are used as the reinforcing materials requiring high strength at similar level. However, the 7xxx-series aluminum alloys, which are Al—Zn—Mg alloys, have poor general corrosion resistance. In addition, these alloys achieve the high strength by allowing MgZn₂ precipitates to distribute in a high density, where the precipitates are precipitates including Zn and Mg. These alloys may therefore possibly cause stress corrosion cracking (the stress corrosion cracking is hereinafter also referred to as “SCC”). The fact is that these alloys are reluctantly subjected to over-aging so as to eliminate or minimize SCC and are used as having a 0.2% yield strength of about 300 MPa. This reduces the features of the alloys as high-strength alloys.

Under such circumstances, various techniques have been proposed on control of chemical composition and/or control of microstructure, such as precipitates, in 7xxx-series aluminum alloy extrusions, so as to allow the aluminum alloy extrusions to have strength and SCC resistance both at excellent levels. In contrast, in 7xxx-series aluminum alloy sheets, there have been conventionally proposed very few techniques for control of chemical composition and control of microstructure, such as precipitates, because few 7xxx-series aluminum alloy sheets have been applied to the uses as above.

Among the techniques, the technique disclosed in Patent literature (PTL) 1 relates to a 7xxx-series aluminum alloy sheet obtained after subjecting a molten metal sequentially to rapid solidification, cold rolling, and further artificial aging. In the 7xxx-series aluminum alloy sheet, intergranular precipitates have a size (in terms of diameter of a circle having an equivalent area) of 3.0 μm or less and have an average area fraction of 4.5% or less, as measured with an optical microscope at 400-fold magnification. Thus, the technique allows the aluminum alloy sheet to have higher strength and better elongation.

With the techniques disclosed in PTL 2 and PTL 3, a 7xxx-series sheet for structural components is prepared by subjecting an ingot to forging, and then to rolling repeatedly in a warm working region for refinement of the microstructure. This technique is intended to allow the 7xxx-series sheet to have higher strength and better SCC resistance. The refinement of the microstructure is performed so as to restrain large angle grain boundaries having a misorientation of 20° or more and to give a metallic texture including small angle grain boundaries having a misorientation of 3° to 10° and being present in an amount of 25% or more, where the large angle grain boundaries cause a potential difference between the grain boundaries and grain insides, where the potential difference in turn causes SCC resistance reduction. However, the repeated warm rolling is performed because hot rolling and/or cold rolling according to a common procedure fails to give such a metallic texture including small angle grain boundaries in an amount of 25% or more. Accordingly, the technique is significantly different in process (steps) from the common procedure and is not considered as a practical technique for producing a target sheet.

CITATION LIST Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2009-144190

PTL 2: JP-A No. 2001-335874

PTL 3: JP-A No. 2002-241882

SUMMARY OF INVENTION Technical Problem

As described above, various proposals have been made in the field of extrusions on control of chemical composition and/or control of microstructure such as precipitates or metallic texture, in order to give 7xxx-series aluminum alloys having strength and SCC resistance both at excellent levels. However, the fact is that there are few proposals on rolled sheets produced according to a common procedure such as sequential performing of ingot soaking, hot rolling, and cold rolling, except special rolling techniques or special production processes, such as clad sheets, rapid solidification technique, and warm rolling technique.

The extrusions significantly differ from the rolled sheets in production process such as hot working step and in the resulting microstructure such as grains and precipitates. For example, the extrusions have a microstructure in which grains are fibrous and extend or elongate in the extrusion direction, whereas the rolled sheets basically have equiaxed grains as grains. It is unknown that the proposals on control of chemical composition and control of microstructure such as precipitates in the extrusions are applicable also to 7xxx-series aluminum alloy sheets and to automobile structural components formed from the 7xxx-series aluminum alloy sheets and are really effective for both higher siren better SCC resistance. Namely, these are a matter of speculation unless actually verified or determined.

Under present circumstances, therefore, there is not yet an effective means, is many unknown points, and is susceptible to clarifying on an effective technique for controlling microstructure of the 7xxx-series aluminum alloy sheets produced according to the common procedure so as to allow the aluminum alloy sheets to have strength and SCC resistance both at excellent levels. Further, the addition of Zn to allow the potential to be less noble is involved in the general corrosion resistance, and the 7xxx aluminum alloy sheets are required to have s lower Zn content. Such low-content Zn contributes to better corrosion resistance and better formability such as bendability, which is a required property in the structural components. However, Zn, if present in a lower content, causes lower strength, which is inconsistent with the requirements for higher strength and causes technical difficulty.

In consideration of these problems, the present invention has an object to provide a 7xxx-series aluminum alloy sheet for structural components such as automobile components, where the aluminum alloy sheet is a rolled sheet produced according to the common procedure, has strength and formability both at satisfactory levels even after natural aging at room temperature, and still has excellent corrosion resistance.

Solution to Problem

To achieve the object, the present invention provides an aluminum alloy sheet for structural components as follows. The aluminum alloy sheet is an Al—Zn—Mg alloy sheet having a chemical composition including, in mass percent, Zn in a content of 3.0% to 6.0%, Mg in a content of 2.5% to 4.5%, and Cu in a content of 0.05% to 0.5%, where the Zn content [Zn] and the Mg content [Mg] meet the condition: [Zn]≧−0.3[Mg]+4.5, with the remainder consisting of Al and inevitable impurities. When the sheet is subjected to natural aging at room temperature subsequent to solution treatment and quenching, grain-boundary precipitates in a microstructure of the resulting sheet have an average compositional ratio of Zn to Mg (Zn/Mg) of from 0.5 to 3.0, where the grain-boundary precipitates are observed with a transmission electron microscope at 60000-fold magnification. When the sheet after the natural aging at room temperature subsequent to solution treatment and quenching is further subjected to one of (I) two-stage artificial aging and (II) single-stage artificial aging, intragranular precipitates in a microstructure of the resulting sheet after the artificial aging (I) or (II) have an average compositional ratio of Zn to Mg (Zn/Mg) of from 1.5 to 3.5, where the intragranular precipitates are observed with a transmission electron microscope at 60000-fold magnification. The two-stage artificial aging (I) includes a first-stage heat treatment at a temperature of from 70° C. to 100° C. for 2 hours or longer; and a second-stage heat treatment at a temperature of from 100° C. to 170° C. for 5 hours or longer. The single-stage artificial aging (II) includes a heat treatment at a temperature of from 100° C. to 150° C. for 12 to 36 hours.

Advantageous Effects of Invention

The inventors of the present invention selected a 7xxx-series aluminum alloy sheet having a chemical composition in which the Zn content is decreased for better corrosion resistance, but the Mg content is increased for ensuring strength and formability at certain levels; focused attention on grain-boundary precipitates and intragranular precipitates in the microstructure of the sheet after natural aging at room temperature; and analyzed how the chemical compositions of these precipitates affect the properties.

As a result, the inventors found that the reduction in Zn content (Zn compositional ratio) in the grain-boundary precipitates reduces the amount of Zn to be (uselessly) consumed in the grain-boundary precipitates and allows Zn solute in the matrix to be present in a certain amount, where the gain-boundary precipitates are formed via natural aging at room temperature of the sheet after production. When the Zn solute is allowed to be present in the matrix in a certain amount as above, the amount of Zn, which is necessary to form artificial aging precipitates (intragranular precipitates) upon subsequent artificial aging, can be maximized, and this leads to maximization of BH response (bake hardening response) and balance between strength and formability, even when the Zn content in the alloy chemical composition is lowered.

The inventors also found that reduction of the Zn content (Zn compositional ratio) in intragranular precipitates, which are aging precipitates formed upon artificial aging, reduces the Zn amount necessary for the formation of the intragranular precipitates (precipitated amount) formed via artificial aging. Thus, the amount of precipitated aging precipitates can be maximized, and this leads to maximization of BH response and balance between strength and formability, even when the Zn content in the alloy chemical composition is lowered, where the aging precipitates contribute to strengthening by artificial aging.

By controlling the chemical compositions of the precipitates as above, the present invention allows even a 7xxx-series aluminum alloy sheet having a lowered Zn content to have better BH response and better balance between strength and ductility (formability). The resulting 7xxx-series aluminum alloy sheet can be provided as a 7xxx-series aluminum alloy sheet for structural components, which is a rolled sheet produced according to the common procedure, has strength and formability both at satisfactory levels, and still has corrosion resistance such as SCC resistance at excellent levels. The “formability” is hereinafter also referred to as “forming workability” or “workability”.

The present invention specifies the chemical composition of grain-boundary precipitates formed via natural aging at room temperature. The microstructure measurement with the TEM is therefore performed not on the sheet immediately after the temper (heat treatment) and before natural aging at room temperature, but on the sheet after natural aging at room temperature (standing at room temperature) for 2 weeks or longer as a rough reference and before forming into a structural component and before artificial aging.

DESCRIPTION OF EMBODIMENTS

Some embodiments of the present invention will be illustrated in a specific manner on a condition-by-condition basis

As used herein, the term “aluminum alloy sheet” refers to a 7xxx-series aluminum alloy sheet that is produced via rolling according to the common procedure, in which an ingot is sequentially subjected to soaking, hot rolling, and cold rolling to give a cold-rolled sheet, and the cold-rolled sheet is further subjected to temper such as solution treatment and quenching (temper designation; T4). In other words, the “aluminum alloy sheet” herein does not include sheets produced typically by special rolling techniques such as the techniques disclosed in PTL 2 and PTL 3, in which an ingot is subjected to forging and then repeatedly to warm rolling; or produced by twin-roll casting and other sheet continuous casting techniques devoid of hot rolling.

Further, the “aluminum alloy sheet” in the present invention is a 7xxx-series aluminum alloy sheet produced in the above manner, in which the microstructure of the sheet after natural aging at room temperature is specified, where the aluminum alloy sheet is used as a material aluminum alloy sheet and will be formed into a target structural component. Accordingly, the term “aluminum alloy sheet” as used herein refers to a sheet after subjecting a sheet produced in the above manner to natural aging at room temperature (standing at room temperature) and before forming into a target structural component and before artificial aging.

Aluminum Alloy Chemical Composition

First, the chemical composition of the aluminum alloy sheet according to the present invention will be described below, together with reasons for specifying individual elements. All contents of elements in percent are in mass percent.

The chemical composition of the aluminum alloy sheet according to the present invention constitutes such a precondition as to allow the rolled sheet produced according to the common procedure to have strength and formability both at satisfactory levels and to still have satisfactory corrosion resistance, where these properties are demand properties for use in structural components such as automobile components. These uses are intended uses in the present invention. Accordingly, the chemical composition of the Al—Zn—Mg—Cu 7xxx-series alloy in the present invention is controlled such that the Zn content is decreased for better corrosion resistance, whereas the Mg content is increased for strength at certain level.

From this viewpoint, the chemical composition of the aluminum alloy sheet according to the present invention includes, in mass percent, Zn in a content of 3.0% to 6.0%, Mg in a content of 2.5% to 4.5%, and Cu in a content of 0.05% to 0.5%, where the Zn content [Zn] and the Mg content [Mg] meet the condition: [Zn]≧−0.3[Mg]+4.5, with the remainder consisting of Al and inevitable impurities. The chemical composition may further selectively include at least one element selected from the group consisting of, as transition element(s), Zr in a content of 0.05% to 0.3%, Mn in a content of 0.1 to 1.5%, Cr in a content of 0.05% to 0.3%, and Sc in a content of 0.05% to 0.3%. The chemical composition may further include at least one element selected from the group consisting of Ag in a content of 0.01% to 0.2% and Sn in a content of 0.001% to 0.1%, in addition to, or instead of, the transition elements.

Zn: 3.0% to 6.0%

Zinc (Zn) is an essential alloy element, forms, together with Mg, clusters upon natural aging at room temperature of the produced sheet after the temper, and contributes to better work hardening properties and better formability into a structural component. In addition, Zn forms aging precipitates upon artificial aging subsequent to forming into a structural component, and contributes to higher strength. The aluminum alloy sheet, if having a Zn content less than 3.0%, may have insufficient strength after artificial aging. However, the aluminum alloy sheet, if having an excessively high Zn content greater than 6.0%, may include a larger amount of grain-boundary precipitates MgZn2, become susceptible to grain-boundary corrosion, and have inferior corrosion resistance. To eliminate or minimize this, the Zn content is controlled to be relatively low. The lower limit of the Zn content is 3.0%, and preferably 3.4%. The upper limit of the content is 6.0%, and preferably 4.6%.

Mg: 2.5% to 4.5%

Magnesium (Mg) is an essential alloy element, forms, together with Zn, clusters upon natural aging at room temperature of the produced sheet after temper, and contributes to better work hardening properties and better formability. In addition, Mg forms aging precipitates upon artificial aging after forming into a structural component and contributes to higher strength. The present invention controls the Zn content to be relatively low and, conversely, controls the Mg content to be relatively high for better formability and higher strength. The aluminum alloy sheet, if having a Mg content less than 2.5 mass percent, may have insufficient strength and lower work hardening properties. However, the aluminum alloy sheet, if having a Mg content greater than 4.5 mass percent, may have lower rolling properties and have higher SCC susceptibility. To eliminate or minimize these, the Mg content is controlled to be from 2.5% to 4.5%.

Zn—Mg Balance Expression

The present invention controls the Zn content to be relatively low and controls the Mg content to be relatively high so as to ensure satisfactory formability, higher strength, and satisfactory corrosion resistance, as described above. For this purpose, the Zn content [Zn] (mass percent) and the Mg content [Mg] (mass percent) are controlled to meet the condition specified by the balance expression: [Zn]≧−0.3[Mg]+4.5, and preferably to meet the condition specified by the balance expression: [Zn]≧−0.5[Mg]+5.75.

The aluminum alloy sheet, as having Zn and Mg contents meeting the condition: [Zn]≧−0.3[Mg]+4.5 and when produced by an after-mentioned preferred production process, may allow the structural component after artificial aging to have a 0.2% yield strength of 380 MPa or more The aluminum alloy sheet, when having Zn and Mg contents meeting the preferred condition: [Zn]≧−0.5[Mg]+5.75 and when produced by the preferred production method, may allow the structural component after artificial aging to have a 0.2% yield strength of 400 MPa or more.

The aluminum alloy sheet, if having Zn and Mg content meeting a condition: [Zn]<−0.3[Mg]+4.5 and when the Zn content is controlled to be relatively low, may possibly fail to allow the structural component after artificial aging to have a 0.2% yield strength of 350 MPa or more, even when the Zn and Mg contents are within the specified ranges, or even when the aluminum alloy sheet is produced by the preferred production process. Likewise, the aluminum alloy sheet, when having Zn and Mg contents meeting a condition: [Zn]<0.5[Mg]+5.75, may fail to allow the structural component after artificial aging to have a 0.2% yield strength of 400 MPa or more.

Cu: 0.05% to 0.5%

Copper (Cu) allows the Al—Zn—Mg alloy to have lower SCC susceptibility and better SCC resistance and contributes to better general corrosion resistance. The aluminum alloy sheet, if having a Cu content less than 0.05%, may less effectively have better SCC resistance and better general corrosion resistance. In contrast, the aluminum alloy sheet, if having a Cu content greater than 0.5%, may contrarily have lower properties such as rolling properties and weldability. To eliminate or minimize these, the lower limit of the Cu content is 0.05%, and preferably 0.10%; and the upper limit is 0.5%, and preferably 0.4%.

At least one element selected from the group consisting of Zr in a content of 0.05% to 0.3%, Mn in a content of 0.1 to 1.5%, Cr in a content of 0.05% to 0.3%. Sc in a content of 0.05% to 0.3%

The transition elements Zr, Mn, Cr, and Sc refine grains of the ingot and of the final product and contribute to higher strength. These elements are selectively contained as needed. The aluminum alloy sheet, when containing at least one of these elements Zr, Mn, Cr, and Sc, but in a content less than the lower limit for each element, may fail to have a targeted high strength, due to such an insufficient content. In contrast, The aluminum alloy sheet, when containing at least one of these elements Zr, Mn, Cr, and Sc, but in a content greater than the upper limit for each element, may include coarse constituents and may have lower elongation. To eliminate or minimize these, when the aluminum alloy sheet contains at least one of these elements, the content(s) of the element(s) may be controlled as follows. Of the Zr content, the lower limit may be 0.05%, and preferably 0.08%; and the upper limit may be 0.3%, and preferably 0.2%. Of the Mn content, the lower limit may be 0.1%, and preferably 0.2%; and the upper limit may be 1.5%, and preferably 1.0%. Of the Cr content, the lower limit may be 0.05%, and preferably 0.1%; and the upper limit may be 0.3%, and preferably 0.2%. Of the Sc content, the lower limit may be 0.05%, and preferably 0.1%; and the upper limit may be 0.3%, and preferably 0.2%.

At least one of Ag in a content of 0.01% to 0.2% and Sn in a content of 0.001% to 0.1%

Silver (Ag) and tin (Sn) allow aging precipitates to be finely precipitated as a result of artificial aging after forming into a structural component, where the fine aging precipitates contribute to higher strength. Thus, these elements effectively promote increase in strength (strengthening) and may be selectively contained as needed. The aluminum alloy sheet, when containing one or both of these elements, but having a Sn content less than 0.001% and/or an Ag content less than 0.01%, may fail to enjoy sufficient effects of strengthening. In contrast, the aluminum alloy sheet, if having a Sn content and/or Ag content being excessively high, may contrarily suffer from reduction in properties such as rolling properties and weldability. In addition, the elements, if present in excess, may offer saturated effects of strengthening. Among them, Ag in an excessively high content merely causes expensiveness. To eliminate or minimize these, the Ag content may be controlled to be from 0.01% to 0.2%, and the Sn content may be controlled to be from 0.001% to 0.1%.

Other Elements

Other elements than the above-mentioned elements are basically inevitable impurities. Such impurity elements may be contained within the ranges prescribed in Japanese Industrial Standards (JIS) on 7xxx-series alloys, assuming (accepting) that aluminum alloy scrap is used as a raw material for melting, in addition to pure aluminum ingots. For example, Ti and B are impurities in a rolled sheet, but these elements effectively contribute to refinement of ingot grains. Thus, the upper limit of the Ti content may be controlled to be 0.2%, and preferably 0.1%, and the upper limit of the B content may be controlled to be 0.05%, and preferably 0.03%. Fe and/or Si, when present each in a content of 0.5% or less, does not affect the properties of the aluminum alloy rolled sheet according to the present invention and may be contained therein.

Microstructure

The microstructure of the 7xxx-series aluminum alloy sheet according to the present invention is specified in a cold-rolled sheet after the production via temper and after natural aging at room temperature for 2 weeks or longer, on the precondition that the aluminum alloy sheet has the above-mentioned alloy chemical composition. Accordingly, the cold-rolled sheet after solution treatment and quenching (temper) is naturally aged at mom temperature for 2 weeks or longer to have a temper T4 microstructure. Then, the average compositional ratio of Zn to Mg (Zn/Mg) in grain-boundary precipitates (natural aging precipitates) in the microstructure is specified, where the grain-boundary precipitates are observed with a transmission electron microscope at 60000-fold magnification. The sheet after subjected to natural aging at room temperature for 2 weeks or longer subsequent to solution treatment and quenching is further subjected to one of the artificial aging (I) and the artificial aging (II), and, in the microstructure of the resulting sheet, the average compositional ratio of Zn to Mg (Zn/Mg) of intragranular precipitates is specified, where the intragranular precipitates are observed with a transmission electron microscope at 60000-fold magnification.

The artificial aging (I) is a two-stage artificial aging including a first-stage heat treatment at a temperature of from 70° C. to 100° C. for 2 hours or longer, and a second-stage heat treatment at a temperature of from 100° C. to 170° C. for 5 hours or longer.

The artificial aging (II) is a single-stage artificial aging including a heat treatment at a temperature of from 100° C. to 150° C. for 12 to 36 hours.

Chemical Composition of Grain-Boundary Precipitates

The present invention specifies the average compositional ratio of Zn to Mg (Zn/Mg) in the grain-boundary precipitates (natural aging precipitates) in the Microstructure of the temper T4 sheet to be from 0.5 to 3.0, where the grain-boundary precipitates are observed with a transmission electron microscope at 60000-fold magnification.

Control of the average compositional ratio of Zn to Mg (Zn/Mg) in the grain-boundary precipitates, which are natural aging precipitates, within the range allows the grain-boundary precipitates (natural aging precipitates) to have a lower Zn content; (Zn compositional ratio) and to have a relatively higher Mg content. This can reduce the amount of Zn to be (uselessly) consumed as the grain-boundary precipitates, but increase the amount of Zn solute in the matrix to a certain level. This maximizes the amount of Zn necessary for the formation of aging precipitates upon the subsequent artificial aging, and maximizes BH response and balance between strength and formability.

The present invention specifies the chemical composition of the grain-boundary precipitates, which are formed by natural aging at room temperature, as described above. Thus, the TEM measurement (observation) of the microstructure is performed not on the sheet immediately after temper such as solution treatment and quenching and before natural aging at room temperature, but on the sheet after natural aging at room temperature (standing at room temperature) for 2 weeks or longer as a rough reference, before forming into a structural component and before artificial aging. In the sheet after the temper and the subsequent natural aging at room temperature (standing at room temperature) for 2 weeks or longer, the compositional ratios in the grain-boundary precipitates change trivially with time, even by natural aging at room temperature proceeding with time thereafter.

The grain-boundary precipitates, if having an average compositional ratio of Zn to Mg (Zn/Mg) greater than the upper limit 3.0, i.e., greater than the specified range, do not differ much in chemical composition from grain-boundary precipitates (natural aging precipitates) including Zn in a large amount and obtained according to conventional techniques. When the Zn content in the alloy chemical composition is lowered, such grain-boundary precipitates having a high Zn content (Zn compositional ratio) uselessly consume a larger amount of Zr, and this excessively reduces the Zn solute amount in the matrix and causes the aluminum alloy sheet to fail to surely have a necessary solute amount. As a result, the amount of Zn is reduced, where Zn is necessary for the formation of aging precipitates upon subsequent artificial aging, and the balance between strength and formability is impaired.

In contrast, if the grain-boundary precipitates have an average compositional ratio of Zn to Mg (Zn/Mg) less than the lower limit 0.5, i.e., lower than the specified range, it means that the Zn content itself in the chemical composition of the aluminum alloy sheet is insufficient. The aluminum alloy sheet therefore has lower BH response and loses the significance of being 7xxx-series aluminum alloy.

Chemical Composition of Intragranular Precipitates

In addition to the grain-boundary precipitates, the present invention specifies the chemical composition of artificial aging precipitates so as to allow the cold-rolled sheet after production via temper to have BH response at certain level. This chemical composition is specified as the microstructure of the sheet after the temper, which is further subjected sequentially to the natural aging at room temperature for 2 weeks or longer and to artificial aging under specific conditions. To this end, the temper T4 sheet is subjected to one of artificial aging (I) and artificial aging (II), and, in the microstructure of the resulting sheet, intragranular precipitates (artificial aging precipitates) are specified to have an average compositional ratio of Zn to Mg (Zn/Mg) of from 1.5 to 3.5, where the intragranular precipitates are observed with a transmission electron microscope at 60000-fold magnification.

The artificial aging (I) is a two-stage artificial aging including a first-stage heat treatment at a temperature of from 70° C. to 100° C. for 2 hours or longer, and a second-stage heat treatment at a temperature of from 100° C. to 1.70° C. for 5 hours or longer.

The artificial aging (II) is a single-stage artificial aging including a heat treatment at a temperature of from 100° C. to 150° C. for 12 to 36 hours.

Control of the average compositional ratio of Zn to Mg (Zn/Mg) in the intragranular precipitates, which are artificial aging precipitates, within the range allows the intra granular precipitates (as artificial aging precipitates) to have a lower Zn content. This can reduce the Zn content (Zn compositional ratio) in the intragranular precipitates and reduce the amount of Zn which is necessary for the formation of the artificial aging precipitates (precipitated amount), even when the Zn content in the alloy chemical composition is lowered. This in turn maximizes the precipitated amount of aging precipitates and maximizes BH response and balance between strength and formability, even at a lower Zn content in the alloy chemical composition, where aging precipitates contributes to strengthening upon artificial aging.

The intragranular precipitates undergo a large change in compositional ratio with time, where the change is caused by natural aging at room temperature with time after the sheet production (after temper). Accordingly, the measurement of the average compositional ratio of Zn to Mg (Zn/Mg) in the intragranular precipitates is performed on a sheet obtained by subjecting the same sheet on which the grain-boundary precipitates measurement is performed (i.e., the sheet obtained by subjecting the sheet after temper to natural aging at room temperature) to the artificial aging under specific conditions. The measurement may be performed in the above manner in view of reproducibility. The artificial aging conditions naturally somewhat affect the number density and the average compositional ratio of Zn to Mg (Zn/Mg) of the intragranular precipitates. Accordingly, the artificial aging in the present invention is preferably performed as a two-stage artificial aging under such conditions that the sheet is heated at one-point rate of temperature rise of 30° C./min up to 90° C., held at that temperature for 3 hours, further heated at a rate of temperature rise of 30° C./min up to 140° C., and held at that temperature for 8 hours.

In contrast, the intragranular precipitates, if having an average compositional ratio of Zn to Mg greater than the upper limit 3.5, i.e., greater than the specified range, do not differ much in chemical composition from intergranular precipitates including Zn in a large amount and obtained by the conventional techniques. When the Zn content in the alloy chemical composition is lowered, the amount of Zn necessary for the formation of aging precipitates (precipitated amount) upon artificial aging is increased because of the high Zn content (Zn compositional ratio) in the intragranular precipitates, and the intragranular precipitates consume a larger amount of Zn upon formation. As a result, aging precipitates, which contribute to strengthening, are precipitated in a smaller amount upon artificial aging, and this impairs BH response and balance between strength and formability.

In contrast, if the intragranular precipitates have an average compositional ratio of Zn to Mg (Zn/Mg) less than the lower limit 1.5, i.e., less than the specified range, it means that the Zn content itself in the chemical composition of the aluminum alloy sheet is insufficient. As a result, the intragranular precipitates, which are artificial aging precipitates, are reduced in amount, and the aluminum alloy sheet has lower BH response and loses the significance of being a 7xxx-series aluminum alloy.

The conditional control of the grain-boundary and intragranular precipitates contributes to better balance between strength and ductility (formability) even of a 7xxx-series aluminum alloy sheet having a lower Zn content. The conditional control provides a 7xxx-series aluminum alloy sheet for structural components, where the aluminum alloy sheet is a rolled sheet produced according to the common procedure, has both a 0.2% yield strength (BH response) of 380 MPa or more, and preferably 400 MPa or more and satisfactory press forming workability, and offers excellent corrosion resistance such as SCC resistance.

Chemical Composition Measurements of Grain-Boundary Precipitates and Intragranular Precipitates

As used herein, the term “grain-boundary precipitates” observed with a TEM (transmission electron microscope) at 60000-fold magnification refers to amorphous precipitates that are observed as lying scattered, but aligned at grain boundaries, can be observed with the TEM, and each have an equivalent circle diameter of about 10 to about 200 nm, where the average compositional ratio of Zn to Mg of the precipitates can be quantitatively determined and analyzed by TEM-EDX (transmission electron microscope-energy dispersive X-ray spectroscope). The “equivalent circle diameter” refers to the diameter of a circle having an equivalent area to that of a target amorphous compound (precipitate). The technique using the equivalent circle diameter has been conventionally widely used as a technique for measuring or specifying the size of the compound accurately and reproducibly.

Also as used herein, the term “intragranular precipitates” observed with a TEM at 60000-fold magnification refers to amorphous precipitates that lie scattered in the grains, can be observed with the TEM, and each have an equivalent circle diameter of about 1 to about 50 nm, where the average compositional ratio of Zn to Mg of the precipitates can be quantitatively determined or analyzed by TEM-EDX

Coarse grain-boundary or intragranular precipitates having an equivalent circle diameter greater than the range in size may significantly adversely affect basic mechanical properties and quality of the sheet. To eliminate or minimize this, the sheet is produced according to a common procedure by such a production method with such quality control as to minimize the coarse precipitates. Accordingly, a TEM measurement range targeting the coarse precipitates is meaningless, and the coarse precipitates are excluded from the measurement object. In contrast, gain-boundary or intragranular precipitates having an equivalent circle diameter less than the range in size may be below the detection limit of the TEM and may often cause errors in analysis of the compositional ratio of Zn to Mg by EDX because of their excessively small sizes. Thus, such excessively small precipitates are also excluded from the measurement object.

‘TEM-EDX’ for use herein refers to an X-ray spectrometer generally attached to the TEM used in the present invention, is well known as an analyzer by energy dispersive X-ray spectroscopy, and is generally referred to as “EDX”. The X-ray spectrometer is widely used for identification and quantitative analysis typically of chemical compositions of compounds (precipitates) observed with a TEM. Also using the X-ray spectrometer, the average compositional ratios of Zn to Mg of grain boundary precipitates and intragranular precipitates observed with the TEM are calculated in the present invention.

The term “average compositional ratio of Zn to Mg” of each of the grain boundary precipitates and intragranular precipitates as specified herein refers to a value obtained by measuring the compositional ratio at any 10 points (sampling 10 samples) at one-fourth depth in the sheet thickness direction from the surface of a test sample (sheet) as a measurement object, and averaging the measurement results. More specifically, the measurement is performed on a cross section perpendicular to the sheet thickness direction of the test sample, where the (TOSS section is a plane passing through any point at one-fourth depth from the surface in the sheet thickness direction and is in parallel with the sheet surface. The measurement is performed using a TEM (transmission electron microscope) at 60000-fold magnification. Specimens are prepared by mechanically polishing each of the 10 samples sampled at the positions in the sheet cross section to shave off about 0.25 mm from the sheet surface, and further performing buffing to condition the surface. Next, the grain boundary precipitates and intragranular precipitates in the view field are identified with an automatic analyzer using backscattered election images, and the average compositional ratios of Zn to Mg of these precipitates are measured, and averages of which are calculated. The measurement is performed at the polished surface of each specimen in a measurement area per each specimen of 240 μm by 180 μm.

Production Method

A method for producing a 7xxx-series aluminum alloy sheet in the present invention will be specifically illustrated below.

The production in the present invention may be performed by a production method according to a common production process for a 7xxx-series aluminum alloy sheet. Specifically, first; a hot-rolled aluminum alloy sheet having a thickness of 1.5 to 5.0 mm is prepared via common production process including casting (e.g., direct chill casting (DC casting) or continuous casting), soaking, and hot rolling. Next, the hot-rolled aluminum alloy sheet is cold-rolled to give a cold-rolled sheet having a thickness of 3 mm or less. In this process, process annealing may be selectively performed one or more times during the cold rolling.

Melting and Casting Cooling Rate

First, in the melting-casting step, a molten aluminum alloy adjusted to have a chemical composition within the 7xxx-series chemical composition range is cast by a common melting casting technique. The melting casting technique may be selected as appropriate typically from continuous casting and semicontinuous casting (direct chill casting (DC casting)).

Soaking

Next, the cast aluminum alloy ingot is subjected to soaking prior to hot rolling. The soaking is performed so as to homogenize the microstructure, namely, so as to eliminate or minimize intragranular segregation in the ingot microstructure.

However, the present invention controls the produced sheet to have such a microstructure as to have average compositional ratios of Zn to Mg in the grain boundary precipitates and intragranular precipitates within the ranges, where the precipitates are observed with the TEM. This is done so as to offer both better formability into a structural component and higher strength after the forming and subsequent artificial aging. The soaking is therefore preferably performed by a two-stage or double soaking process. The soaking, if performed as common single soaking or single-stage soaking, may less allow the sheet microstructure after the natural aging at room temperature subsequent to the temper to have average compositional ratios of Zn to Mg in the grain boundary precipitates and intragranular precipitates within ranges specified in the present invention.

In the two-stage soaking, a workpiece after first-stage soaking is cooled not down to 200° C. or lower, but down to a cooling end temperature of higher than 200° C., held at that temperature, and subjected to hot rolling as being held at that temperature or as being reheated to a higher temperature. In contrast in the double soaking, a workpiece after first soaking is once cooled down to a temperature of 200° C. or lower (including room temperature), reheated, and held at the reheating temperature for a predetermined time, followed by hot rolling start.

In the two-stage or double soaking process, the first-stage or first soaking is intended to finely disperse zinc (Zn) compounds and transition element compounds and to perform refinement of such compounds that affect the formability of the aluminum alloy sheet into a structural component; and the second-stage or second soaking is intended to accelerate the solid-solution (dissolution) of Zn, Mg, and Cu. This allows the grain boundary precipitates and intragranular precipitates to have average compositional ratios of Zn to Mg within the specified ranges, where the precipitates are observed with the TEM.

To achieve the intensions, the first-stage or first soaking temperature may be controlled to be from 400° C. to 450° C., and preferably from 400° C. to 440° C. The ingot is heated up to and held at a temperature within this range. The first-stage or first soaking, if performed at a temperature lower than 400° C., may fail to give sufficiently effective refinement of the compounds. In contrast, the first-stage or first soaking, if performed at a temperature higher than 450° C., may cause the compounds to coarsen. Holding in the first-stage or first soaking may be performed for a holding time of about 1 to about 8 hours.

The second-stage or second soaking temperature may be controlled in the range of from 450° C. to the solidus temperature, and preferably in the range of from 470° C. to the solidus temperature. The heating and holding of the ingot at a temperature within the range may accelerate the solid-solution (dissolution) of the compounds (elements). The second-stage or second soaking, if performed at a temperature lower than 450° C., may fail to allow these elements to be sufficiently dissolved. In contrast, the second-stage or second soaking, if performed at a temperature higher than the solidus temperature, may cause partial melting to impair mechanical properties. To eliminate or minimize this, the temperature is controlled to be equal to or lower than the solidus temperature in terms of upper limit. The holding in the second-stage or second soaking may be performed for a holding time of from about 1 to about 8 hours.

Hot Rolling

The hot rolling, if started at a temperature higher than the solidus temperature, may cause burning and may be performed with difficulty in itself. In contrast, the hot rolling, if started at a temperature lower than 350° C., may require an excessively large load and may be performed with difficulty in itself. To eliminate or minimize these, the hot rolling may be performed at a start temperature selected within the range of from 350° C. to the solidus temperature and may give a hot-rolled sheet having a thickness of about 2 to about 7 mm. The annealing (heat treatment) of the hot-rolled sheet prior to cold rolling is not necessary.

Cold Rolling

In the cold rolling, the hot-rolled sheet is rolled to give a cold-rolled sheet (including a coil) having a desired final thickness of from about 1 to about 3 mm. The workpiece may be subjected to process annealing between cold rolling passes.

Solution Treatment

The workpiece (cold-rolled sheet) after cold rolling is subjected to solution treatment as temper. This allows, in particular, the grain-boundary precipitates (natural aging precipitates) to have an average compositional ratio of Zn to Mg within the specified range, where the precipitates are observed with the TEM. The solution treatment may be performed as heating and cooling in a common continuous heat treatment line, and the procedure of which is not limited. However, the solution treatment is preferably performed by heating the workpiece to a solution treatment temperature of from 450° C. to the solidus temperature, and more preferably from 480° C. to 550° C., and holding the workpiece for a duration in the range of from 2 or 3 seconds to 30 minutes after the temperature reaches the predetermined solution treatment temperature. This is preferred for obtaining sufficient amounts of individual elements as solutes and for grain refinement.

Cooling (temperature fall) after the solution treatment may be performed at a cooling rate as high as possible, e.g., 10° C./s or more, preferably 30° C./s or more, and more preferably 40° C./s or more. Such cooling at a high cooling rate may allow, in particular, the grain-boundary precipitates (natural aging precipitates) to have an average compositional ratio of Zn to Mg within the specified range, where the precipitates are observed by the TEM. The cooling (temperature fall) after the solution treatment, if performed at a low cooling rate, may lead to the formation of coarse grain-boundary precipitates and may less allow, in particular, the grain-boundary precipitates (natural aging precipitates) to have an average compositional ratio of Zn to Mg within the specified range, where the precipitates are observed with the TEM.

Accordingly, the cooling after the solution treatment may be performed by forced cooling means or by quenching the workpiece in warm water at a temperature of room temperature to 100° C. The forced cooling means may be selected from, alone or in combination, air cooling means such as fan; and water cooling means such as mist, spray, and immersion. In this connection, the solution treatment may be basically performed only once. However, typically when natural aging at room temperature excessively proceeds, solution treatment and/or reversion treatment may be performed again under the preferred conditions so as to temporarily cancel the excessively proceeded natural aging at room temperature. This is preferred for surely offering satisfactory formability into an automobile component.

In an embodiment, the aluminum alloy sheet according to the present invention is used as a material, formed into an automobile component, and assembled as the automobile component. The aluminum alloy sheet may be formed into an automobile component, then further subjected to artificial aging, and used as an automobile component or automobile body.

Artificial Aging

The 7xxx-series aluminum alloy sheet according to the present invention, after formed into a structural component, is subjected to artificial aging treatment (hereinafter also simply referred to as “artificial aging” or “aging”). The artificial aging allows, in particular, the intragranular precipitates (artificial aging precipitates) to have an average compositional ratio of Zn to Mg within the specified range, where the precipitates are observed with the TEM. Thus, the resulting structural component may have a desired strength as structural components such as automobile components, in terms of 0.2% yield strength of 380 MPa or more, and preferably 400 MPa or more. The artificial aging is preferably performed at the time point after the forming of the material 7xxx-series aluminum alloy sheet into an automobile component (structural component). This is because a 7xxx-series aluminum alloy sheet after artificial aging has high strength, but has lower formability and may hardly undergo forming when the target automobile component has a certain complicated shape.

The artificial aging conditions such as temperature and time may be determined within common artificial aging conditions (T6 or T7), depending typically on the desired strength, the strength of the material 7xxx-series aluminum alloy sheet, and/or how extent the natural aging at room temperature proceeds. In this connection, exemplary artificial aging conditions are as follows. The artificial aging, when performed as single-stage aging, may be performed as aging at 100° C. to 150° C. for 12 to 36 hours (including the over-aging region). The artificial aging, when performed as two-stage aging, may be performed as a first-stage heat treatment at a temperature of from 70° C. to 100° C. for 2 hours or longer; and as a second-stage heat treatment at a temperature of from 100° C. to 170° C. for 5 hours or longer (including the over-aging region).

Of the artificial aging conditions within the ranges, there are preferred conditions so as to allow, in particular, the intragranular precipitates (artificial aging precipitates) to have an average compositional ratio of Zn to Mg within the specified range, where the precipitates are observed with the TEM. The preferred conditions are also intended to allow the article after BH (bake hardening) to surely have a strength of 380 MPa or more, and preferably 400 MPa or more, where the strength is required as a structural component. The preferred conditions are such conditions that heating is performed at a rate of temperature rise of 30° C./min or less and as low as possible, where the rate of temperature rise herein is a rate of temperature rise upon artificial aging in the single-stage artificial aging process; or a rate of temperature rise upon first-stage artificial aging in the two-stage artificial aging process. This configuration can reduce the Zn content in the formed intragranular precipitates (artificial aging precipitates), but ensure the Zn amount in the matrix so as to offer the average compositional ratio of Zn to Mg within the specified range. The rate of temperature rise in the second-stage artificial aging does not approximately affect the strength after BH and may be selected from efficient rates of temperature rise of 30° C./min or more. The second-stage reheating does not approximately affect the strength even when the reheating is performed after the workpiece underwent holding at the first-stage heating temperature is once cooled down to room temperature, or even when the reheating is performed successively from the holding temperature. Either reheating procedure will do.

The artificial aging conditions specified in the present invention as preferred conditions so as to allow the intragranular precipitates to have an average compositional ratio of Zn to Mg (Zn/Mg) within the range are as follows. Under the specific conditions, the artificial aging is performed as a two-stage process in which the workpiece is heated at a rate of temperature rise of 30° C./min up to 90° C., held at that temperature for 3 hours, further heated at a rate of temperature rise of 30° C./min up to 140° C., and held at that temperature for 8 hours. These conditions are specified so as to have a correlation with the average compositional ratio of Zn to Mg in intragranular precipitates, which are formed in the actual artificial aging after forming of the material 7xxx-series aluminum alloy sheet into an automobile component and which affect the strength.

Naturally, the conditions for actual artificial aging after the forming of the material 7xxx-series aluminum alloy sheet into an automobile component may vary differently. However, the artificial aging conditions, when being artificial aging conditions within the preferred ranges, do not significantly affect the formed amount and the average compositional ratio of Zn to Mg of the intragranular precipitates, when only the rate of temperature rise is controlled to be 30° C./rain or less in the single-stage aging or in the first-stage aging′ in the two-stage artificial aging, where the amount and the average compositional ratio affect the strength. In addition, the rate of temperature rise of 30° C./min in the artificial aging conditions is a minimum condition to offer a desired strength of 380 MPa or more, which strength is necessary as a structural component. The rate of temperature rise is specified in the present invention in relation with the average compositional ratio of Zn to Mg (Zn/Mg) in the intragranular precipitates. Specifically, artificial aging, when performed under conditions within the preferred ranges, may allow the resulting component to have increasing strength with a decreasing rate of temperature rise of less than 30° C./min. Thus, the resulting average compositional ratio of Zn to Mg may have a correlation with the average compositional ratio of Zn to Mg in intragranular precipitates formed in actual artificial aging subsequent to forming into an automobile component.

EXAMPLES

With varying the average compositional ratios of Zn to Mg (Zn/Mg) in the specific grain boundary precipitates and intragranular precipitates, 7xxx-series aluminum alloy cold-rolled sheets having Al—Zn—Mg—Cu chemical compositions given in Table 1 were produced. The prepared cold-rolled sheets were naturally aged at room temperature for 2 weeks subsequent to solution treatment and quenching (temper T4). The resulting sheets were subjected to measurements of the average compositional ratio of Zn to Mg in grain-boundary precipitates, work hardening coefficient n (10% to 15%), and mechanical properties such as strength and elongation. The sheets after natural aging at room temperature for 2 weeks were further subjected to artificial aging (T6) and then subjected to evaluations of the average compositional ratio of Zn to Mg in intragranular precipitates; mechanical properties such as strength; and general corrosion resistance. Results of these measurements and evaluations are presented in Tables 2 and 3.

The average compositional ratios of Zn to Mg (Zn/Mg) in the grain boundary precipitates and intragranular precipitates were controlled by varying the chemical compositions given in Table 1 together with varying the ingot soaking conditions and the average cooling rates of the cold-rolled sheets after solution treatment, as given in Tables 2 and 3.

Specifically, commonly in each sample, a 7xxx-series molten aluminum alloy having the chemical composition given in Table 1 was subjected to direct chill casting (DC casting) to give an ingot having a thickness of 45 mm, a width of 220 mm, and a length of 145 mm. The ingot was subjected to two-stage soaking or double soaking under the conditions given in Table 2. In the two-stage soaking, the workpiece after first soaking as cooled down to 250° C., the cooling was once stopped at that temperature, and the workpiece was reheated up to the second-stage soaking temperature, held at that temperature, and cooled down to the hot rolling start temperature, followed by hot rolling start. In the double soaking, the workpiece after first soaking was once cooled down to room temperature, reheated up to, and held at the second soaking temperature, and cooled down to the hot rolling start temperature, followed by hot rolling start. In single soaking as given in Table 2, the workpiece was held at the soaking temperature for the soaking time according to a common procedure without once-cooling and subsequent second reheating, and the resulting workpiece was cooled down to the hot rolling start temperature, followed by hot rolling start.

Each workpiece after the soaking as mentioned above was subjected to hot rolling at the start temperature given in Table 2 and yielded a hot-rolled sheet having a thickness t of 5 mm. The hot-rolled sheet was subjected to cold rolling to a thickness t of 2 mm, without performing a heat treatment (rough annealing). In common in each sample, the cold-rolled sheet was subjected to a solution treatment at 500° C. for 1 min. The sheet after the solution treatment was cooled down to room temperature by forced wind cooling in a different manner at a different average cooling rate to give a T4 temper aluminum alloy sheet. In common in each sample, the aluminum alloy sheet after the solution treatment was naturally aged at room temperature for 2 weeks. Sheet-like test specimens were sampled from the resulting sheet, and subjected to measurement of average compositional ratio of Zn to Mg (Zn/Mg) in the grain boundary precipitates, measurement of work hardening coefficient n; and determinations of mechanical properties such as strength and elongation according to procedures mentioned below.

The aluminum alloy sheet after the natural aging at room temperature for 2 weeks was subjected to two-stage artificial aging as T6 temper. In the artificial aging, the sheet was heated at a rate of first-stage temperature rise, and held at a first-stage aging temperature for 3 hours, where the rate of temperature rise and aging temperature are given in Tables 2 and 3. Thereafter, the sheet was reheated successively from the holding temperature at a rate of temperature rise of 30° C./min up to 140° C., and held at 140° C. for 8 hours. The two-stage artificial aging was performed as simulating artificial aging after forming of the sheet into an automobile component.

Some of examples (examples according to the present invention) subjected to the heat treatments in which only the T6 conditions were positively varied so as to examine how the T6 conditions affect the properties. Specifically, Example 18 in Table 2 was subjected to two-stage artificial aging, in which the first-stage heat treatment was performed by heating the sample at a rate of temperature rise of 30° C./min up to 90° C. and holding the sample at 90° C. for 3 hours; and the second-stage heat treatment was performed by heating the sample at a rate of temperature rise of 30° C./min up to 130° C., and holding the sample at 130° C. for 12 hours. Example 19 in Table 2 was subjected to single-stage artificial aging by heating the sample at a rate of temperature rise of 30° C./min up to 120° C., and holding the sample at 120° C. for 24 hours. WS.

Sheet-like test specimens were sampled from the central part of the aluminum alloy sheet after the artificial aging to examine the average compositional ratio of Zn to Mg (Zn/Mg) intragranular precipitates; mechanical properties; and corrosion resistance, according to procedures mentioned below. Results of the examinations are presented in Tables 2 and 3.

Chemical Composition Measurements on Grain-Boundary Precipitates and Intragranular Precipitates

The average compositional ratios of Zn to Mg in the grain-boundary precipitates and in the intragranular precipitates were individually measured using a TEM-EDX at 60000-fold magnification according to the above-mentioned procedures.

Mechanical Properties

Each sheet-like test specimen was subjected commonly to a mom-temperature tensile test in a direction perpendicular to the rolling direction of the test specimen to measure 0.2% yield strength (MPa) and total elongation (%) as the mechanical properties. The mom-temperature tensile test was performed at room temperature of 20° C. according to Japanese Industrial Standards JIS 2241:1980. The test was performed at a constant tensile speed of 5 mm/min until the test specimen ruptured.

Work Hardening Coefficient n

The work hardening coefficient n was measured in the following manner. The sheet-like test specimen after the artificial aging was processed into a JIS No. 5 tensile test specimen with a gauge length of 50 mm and subjected to a room-temperature tensile test in a direction perpendicular to the rolling direction. A true stress and a true strain were calculated from the endpoint of yield elongation, plotted on a logarithmic scale with the abscissa indicating the strain and the ordinate indicating the stress. The gradient of a straight line indicated by measurement points was calculated between two points of nominal strain of 10% and 15%, and this was defined as the work hardening coefficient n (10% to 15%).

Grain-Boundary Corrosion Susceptibility

For evaluation of general corrosion resistance, the sheet-like test specimens (three test specimens) after the artificial aging were subjected to a grain-boundary corrosion susceptibility test in conformity with the standard prescribed in old JIS-W1103. The test was performed under such conditions as follows. First, the surface of the test specimen was cleaned by immersing the test specimen sequentially in a nitric acid aqueous solution (30 mass percent) at room temperature for 1 minute, in a sodium hydroxide aqueous solution (5 mass percent) at 40° C. for 20 seconds, and in a nitric acid aqueous solution (30 mass percent) at room temperature for one minute. A current at a current density of 1 mA/cm² was supplied for 24 hours while the test specimen was immersed in a sodium chloride aqueous solution (5 mass percent). The test specimen was then retrieved, and a cross section of which was cut out and polished. How deep the test specimen surface was corroded was measured using an optical microscope at 100-fold magnification. A sample having a corrosion depth of 200 μm or less was considered as suffering from slight corrosion and evaluated as “◯”. A sample having a corrosion depth of greater than 200 μm was considered as suffering from significant corrosion and evaluated as “x”.

As demonstrated in Tables 1 and 2, Examples 1 to 19 had aluminum alloy chemical compositions within the ranges specified in the present invention and are produced under conditions within the preferred production conditions.

As a result, when the sheets are naturally aged at room temperature subsequent to solution treatment and quenching (temper T4), grain-boundary precipitates in the microstructure of the resulting sheets have an average compositional ratio of Zn to Mg (Zn/Mg) of from 0.5 to 3.0, where the grain-boundary precipitates are observed with a transmission electron microscope at 60000-fold magnification. When the temper T4 sheets are further subjected to the two-stage artificial aging to give temper T6 sheets, intragranular precipitates in the microstructure of the temper T6 sheets have an average compositional ratio of Zn to Mg (Zn/Mg) of from 1.5 to 3.5, namely, within the range specified for the microstructure in the present invention, where the intragranular precipitates are observed with a transmission electron microscope at 60000-fold magnification.

The results demonstrated that the aluminum alloy sheets, even after natural aging at room temperature for 2 weeks, have a satisfactory work hardening coefficient n of 0.22 or more, have excellent ductility, and offer excellent formability into a structural component. Simultaneously, the aluminum alloy sheets, even after natural aging at room temperature, have excellent BH response and high strength. The examples have a chemical composition meeting the condition: [Zn]≧−0.3[Mg]+4.5, and thereby have a 0.2% yield strength after artificial aging (of temper T6) of 380 MPa or more, which level is necessary for a structural component. Among them, the examples having a chemical composition meeting the condition: [Zn]≧−0.5[Mg]+5.75 have a higher 0.2% yield strength after artificial aging of 430 MPa or more.

The aluminum alloy sheets according to Examples 5, 18, and 19 are produced under such conditions that only T6 conditions are varied so as to determine how the difference in T6 conditions affect the properties. Comparisons among these examples reveal that the examples each have an average compositional ratio of Zn to Mg (Zn/Mg) in the intragranular precipitates of from 1.5 to 3.5, although they have slight differences in the microstructure and properties. In addition, these examples each have a 0.2% yield strength after artificial aging at a level of 420 MP equivalently, and still have good corrosion resistance. This supports the significance of the preferred ranges of temperature and time in the artificial aging, as described in the paragraph [0067].

In contrast, the aluminum alloy sheets according to Comparative Examples 20 to 32 in Table 3 have alloy chemical compositions No. 17 to 29 as given in Table 1, which are out of the ranges s 1 in the present invention.

The aluminum alloy sheets according to Comparative Examples 20 and 21 in Table 3 respectively have alloy chemical compositions Nos. 17 and 18 in Table 1, in which Zn and Mg contents, [Zn] and [Mg], are both within the specified ranges, but the [Zn] and [Mg] meet neither the condition: [Zn]≧−0.3[Mg]+4.5, nor the condition: [Zn]≧−0.5[Mg]+5.75, where the conditions are specified as balance expressions between Zn and Mg. Although being produced under conditions within the preferred production conditions, these aluminum alloy sheets do not meet the conditions for the precipitates compositional ratios, have a work hardening coefficient n (10% to 15%) after natural aging at room temperature at a level of 0.21 to 0.22, but have an excessively low 0.2% yield strength after artificial aging of at highest about 329 MPa. The aluminum alloy sheets fail to have formability and strength both at satisfactory levels.

Among the examples in Table 2, Examples 1 to 5, 7, and 9 have alloy chemical compositions Nos. 1 to 4, 6, and 8 in Table 1, in which the chemical compositions meet the Zn—Mg balance expression: [Zn]≧−0.3[Mg]+4.5, but do not meet the Zn—Mg balance expression: [Zn]≧−0.5[Mg]+5.75. These examples have a 0.2% yield strength of the T6 sheet at a level of 391 to 429 MPa. The 0.2% yield strength level of these examples is relatively lower as compared with the 0.2% yield strength of the T6 sheets at a level of 431 to 459 MPa of the other examples which meet both the balance expressions. The results of these samples support the significance of the balance expressions, by which the Zn content is controlled to be lower, and the Mg content is controlled to be higher.

The aluminum alloy sheets according to Comparative Examples 22 to 24 in Table 3 have alloy chemical compositions Nos. 19 to 21 in Table 1, in which the Mg content is lower than the lower limit and is excessive low. Accordingly, even though meeting the Zn—Mg balance expressions and being produced under conditions within the preferred production conditions, these aluminum alloy sheets have, in particular, a compositional ratio of Zn to Mg (Zn/Mg) in the intragranular precipitates lower than the lower limit. In addition, the aluminum alloy sheets have a work hardening coefficient n after natural aging at room temperature at a level of 0.21, but have an excessively low 0.2% yield strength after artificial aging of at highest of about 353 MPa. Thus, the aluminum alloy sheets fail to have formability and strength both at satisfactory levels.

The aluminum alloy sheets according to Comparative Examples 25 to 28 have alloy chemical compositions Nos. 22 to 25 in Table 1, in which the Mg content is higher than the upper limit and is excessively high. The aluminum alloy sheets, even though meeting the Zn—Mg balance expression(s) and being produced under conditions within the preferred production conditions, have, in particular, a compositional ratio Zn/Mg in the intragranular precipitates of lower than the lower limit. The aluminum alloy sheets have an excessively low 0.2% yield strength after artificial aging of at highest about 369 MPa, although having a work hardening coefficient n (10% to 15%) after natural aging at room temperature at a level of 0.22 to 0.23. The aluminum alloy sheets fail to have formability and strength both at satisfactory levels.

The aluminum alloy sheets according to Comparative Examples 29 and 30 have alloy chemical compositions Nos. 26 and 27 in Table 1, in which the Zn content is higher than the upper limit and is excessively high. The aluminum alloy sheets, even though meeting the Zn—Mg balance expressions and being produced under conditions within the preferred production conditions, have, in particular, a compositional ratio Zn/Mg in the intragranular precipitates of lower than the lower limit. The aluminum alloy sheets have an excessively low 0.2% yield strength after artificial aging of at highest about 362 MPa, although having a work hardening coefficient n (10% to 15%) after natural aging at room temperature at a level of 0.22 to 0.23. The aluminum alloy sheets fail to have formability and strength both at satisfactory levels and have poor corrosion resistance.

The aluminum alloy sheet according to Comparative Example 31 has an alloy chemical composition No. 28 in Table 1, in which the Cu content is lower than the lower limit. The aluminum alloy sheet meets the Zn—Mg balance expressions, is produced under conditions within the preferred production conditions, meets the conditions for the precipitates compositional ratios, has a work hardening coefficient n (10% to 15%) after natural aging at room temperature at a level of 0.23 level, and has a 0.2% yield strength after artificial aging at a level of 448 MPa. Thus, the aluminum alloy sheet has formability and strength both at satisfactory levels. This aluminum alloy sheet, however, fatally has poor corrosion resistance.

The aluminum alloy sheet according to Comparative Example 32 has an alloy chemical composition No. 29 in Table 1, in which the Cu content is higher than the upper limit. The aluminum alloy sheet meets the Zn—Mg balance expressions, is produced under conditions within the preferred production conditions, and meets the conditions for the precipitates compositional ratios. However, the aluminum alloy sheet has a low work hardening coefficient n (10% to 15%) after natural aging at room temperature of 0.209, although having a 0.2% yield strength after artificial aging of 456 MPa level. Thus, the aluminum alloy sheet fails to have formability and strength both at satisfactory levels.

The aluminum alloy sheets according to Comparative Examples 33 to 37 in Table 3 employ the aluminum alloy No. 1 in Table 1, which has a chemical composition meeting the conditions specified in the present invention, but are produced under conditions out of the preferred production conditions range.

Comparative Example 33 undergoes first soaking at an excessively low temperature.

Comparative Example 34 undergoes second soaking at an excessively low temperature.

Comparative Example 35 undergoes cooling after solution treatment at an excessively low average cooling rate.

Comparative Examples 36 and 37 undergo only one soaking (only first soaking).

The aluminum alloy sheets according to these comparative examples produced via soaking under conditions out of the preferred range have a compositional ratio Zn/Mg in the grain-boundary precipitates failing to meet the condition, or have a compositional ratio Zn/Mg in the intragranular precipitates failing to meet the condition. In addition, the aluminum alloy sheets have a work hardening coefficient n (10% to 15%) after natural aging at room temperature of less than (122 and thereby have poor ductility and poor formability into a structural component; or also have a low 0.2% yield strength after artificial aging of less than 340 MPa. Thus, the aluminum alloy sheets fail to have formability and strength both at satisfactory levels.

The results support the critical significances of the conditions specified in the present invention, so as to allow the aluminum alloy sheets according to the present invention to have all of high strength, high ductility (formability), and satisfactory SCC resistance.

TABLE 1 Aluminum alloy chemical composition in mass percent (with the remainder being Al) [Zn] ≧ −0.3 [Zn] ≧ −0.5 [Mg] + [Mg] + Number Zn Mg 4.5 5.75 Cu Zr Mn Cr Sc Ag Sn Si Fe Ti 1 3.8 2.5 ∘ x 0.17 — — — — — — 0.08 0.10 0.02 2 3.5 3.5 ∘ x 0.17 — — — — — — 0.08 0.10 0.02 3 3.2 4.5 ∘ x 0.20 — — — — — — 0.08 0.10 0.02 4 4.45 2.5 ∘ x 0.05 0.15 — — — — — 0.08 0.10 0.02 5 4.55 2.5 ∘ ∘ 0.18 — 0.90 — — — — 0.08 0.10 0.02 6 3.95 3.5 ∘ x 0.18 — — 0.20 — — — 0.08 0.10 0.02 7 4.05 3.5 ∘ ∘ 0.18 — — — 0.15 — — 0.08 0.10 0.02 8 3.45 4.5 ∘ x 0.18 — — — — 0.10 — 0.08 0.10 0.02 9 3.55 4.5 ∘ ∘ 0.50 — — — — — 0.07 0.08 0.10 0.02 10 5.5 2.5 ∘ ∘ 0.50 — — — — — — 0.08 0.10 0.02 11 6 2.5 ∘ ∘ 0.20 — — — — — — 0.08 0.10 0.02 12 4.5 4.5 ∘ ∘ 0.16 — — — — — — 0.08 0.10 0.02 13 5.5 4.5 ∘ ∘ 0.18 0.10 — 0.05 — — — 0.08 0.10 0.02 14 6 4.5 ∘ ∘ 0.15 0.05 — — 0.05 — — 0.08 0.10 0.02 15 6 3.5 ∘ ∘ 0.17 0.14 0.20 0.05 — — 0.005 0.08 0.10 0.02 16 5 3.5 ∘ ∘ 0.16 0.14 0.20 0.05 — 0.02 0.07 0.08 0.10 0.02 17 3.7 2.5 x x 0.18 — — — — — — 0.08 0.10 0.02 18 3.1 4.5 x x 0.17 — — — — — — 0.08 0.10 0.02 19 3.8 2.45 ∘ x 0.17 — — — — — — 0.08 0.10 0.02 20 4.5 2.45 ∘ x 0.17 — — — — — — 0.08 0.10 0.02 21 6 2.45 ∘ ∘ 0.17 — — — — — — 0.08 0.10 0.02 22 3.2 4.55 ∘ x 0.18 — — — — — — 0.08 0.10 0.02 23 3.6 4.55 ∘ ∘ 0.18 — — — — — — 0.08 0.10 0.02 24 4.5 4.55 ∘ ∘ 0.17 — — — — — — 0.08 0.10 0.02 25 6 4.55 ∘ ∘ 0.16 — — — — — — 0.08 0.10 0.02 26 6.1 3 ∘ ∘ 0.18 — — — — — — 0.08 0.10 0.02 27 6.1 4 ∘ ∘ 0.15 — — — — — — 0.08 0.10 0.02 28 5 3.5 ∘ ∘ 0.04 — — — — — — 0.08 0.10 0.02 29 5 3.5 ∘ ∘ 0.6 — — — — — — 0.08 0.10 0.02

TABLE 2 Aluminum alloy sheet after Aluminum alloy sheet natural at room temperature (T4) after artificial aging (T6) Micro- Form- Micro- structure ability structure Compo- Work First-stage Compo- sitional harden- artificial aging sitional Hot ratio ing Mechanical Rate of ratio Gen- Soaking rolling Solution treatment ZnMg in coeffi- properties temper- ZnMg eral Alloy Soak- First Second Start Solution Average grain- cient 0.2% Aging ature in intra- 0.2% corro- Over- number ing Soaking Soaking Soaking Soaking temper- treatment cooling boundary n Yield Elon- temper- rise granular Yield sion all Cate- Num- in pat- temper- time temper- time ature temper- rate preci- (10%- strength gation ature ° C./ preci- strength resis- assess- gory ber Table 1 tern ature ° C. hr ature ° C. hr ° C. ature ° C. ° C./s pitates 15%) MPa % ° C. min pitates MPa tance ment Exam- 1 1 double 430 4 510 2 440 500 10 1.25 0.246 259 23 90 30 2.06 394 ⊚ ◯ ple 2 1 double 430 4 510 2 440 495 25 1.23 0.243 252 23 90 30 2.09 398 ⊚ ◯ 3 2 double 450 2 490 4 450 480 15 0.71 0.271 226 24 80 30 1.74 391 ⊚ ◯ 4 3 double 440 8 470 6 430 490 20 0.54 0.298 238 24 70 30 1.51 382 ⊚ ◯ 5 4 double 410 6 480 4 350 495 25 1.62 0.22 292 23 90 30 2.33 428 ◯ ◯ 6 5 double 400 3 460 8 410 470 30 1.75 0.222 295 23 95 30 2.51 439 ⊚ ⊚ 7 6 double 420 4 490 3 420 505 10 0.9 0.235 297 26 80 30 1.89 429 ⊚ ◯ 8 7 two- 400 5 510 2 400 465 45 0.98 0.232 289 24 70 30 2.01 451 ⊚ ⊚ stage 9 8 two- 450 2 460 6 380 500 20 0.61 0.294 231 24 75 30 1.55 397 ⊚ ◯ stage 10 9 two- 430 6 450 8 370 470 60 0.65 0.306 228 25 70 30 1.58 431 ⊚ ⊚ stage 11 10 double 420 4 470 4 380 470 80 2.48 0.249 261 23 95 30 3.01 442 ◯ ⊚ 12 11 two- 410 8 500 2 460 460 50 2.96 0.252 264 23 90 30 3.32 444 ◯ ⊚ stage 13 12 double 440 4 480 3 420 475 40 0.73 0.295 237 24 70 30 1.7 434 ⊚ ⊚ 14 13 two- 440 4 490 4 440 470 65 0.87 0.237 308 23 70 30 1.81 438 ◯ ⊚ stage 15 14 double 430 6 470 6 430 470 45 0.93 0.242 315 23 75 30 1.87 440 ◯ ⊚ 16 15 two- 450 1 500 1 450 465 70 1.58 0.231 312 22 85 30 2.25 450 ◯ ⊚ stage 17 16 double 440 3 480 4 420 475 50 1.33 0.238 318 22 75 30 2.12 459 ◯ ⊚ 18 4 double 410 6 480 4 350 495 25 1.62 0.22 292 22 90 30 2.26 425 ◯ ◯ 19 4 double 410 6 480 4 350 495 25 1.62 0.22 292 22 120 30 2.21 421 ◯ ◯

TABLE 3 Aluminum alloy sheet after Aluminum alloy sheet natural at room temperature (T4) after artificial aging (T6) Micro- Form- Micro- structure ability structure Compo- Work First-stage Compo- sitional harden- artificial aging sitional Hot Solution ratio ing Mechanical Rate of ratio Gen- Soaking rolling treatment ZnMg in coeffi- properties temper- ZnMg eral Alloy Soak- First Second Start Solution Average grain- cient 0.2% Aging ature in intra- 0.2% corro- Over- number ing Soaking Soaking Soaking Soaking temper- treatment cooling boundary n Yield Elon- temper- rise granular Yield sion all Cate- Num- in pat- temper- time temper- time ature temper- rate preci- (10%- strength gation ature ° C./ preci- strength resis- assess- gory ber Table 1 tern ature ° C. hr ature ° C. hr ° C. ature ° C. ° C./s pitates 15%) MPa % ° C. min pitates MPa tance ment Com- 20 17 double 430 4 510 2 440 470 20 1.38 0.214 238 21 100 30 1.41 268 ∘ x par- 21 18 double 430 4 510 2 440 470 20 0.48 0.227 215 24 100 30 1.43 329 ∘ x ative 22 19 two- 450 2 460 6 380 470 20 1.23 0.216 232 21 90 30 1.37 341 ∘ x Exam- stage x ple 23 20 double 430 4 510 2 440 470 20 2.49 0.215 234 21 95 30 1.42 349 ∘ x 24 21 two- 450 2 460 6 380 470 20 2.92 0.212 238 21 100 30 1.48 353 ∘ x stage x 25 22 double 430 4 510 2 440 470 20 0.47 0.226 221 24 80 30 1.31 333 ∘ x 26 23 double 430 4 510 2 440 470 20 0.64 0.232 227 24 85 30 1.38 346 ∘ x 27 24 double 430 4 510 2 440 470 20 0.75 0.235 233 23 100 30 1.43 353 ∘ x 28 25 two- 450 2 460 6 380 470 20 0.94 0.239 236 23 90 30 1.47 369 ∘ x stage x 29 26 double 430 4 510 2 440 470 20 2.51 0.219 237 22 95 30 1.44 357 x x 30 27 two- 450 2 460 6 380 470 20 3.04 0.23 238 22 100 30 1.45 362 x x stage x 31 28 double 450 2 460 6 380 470 20 1.37 0.237 315 22 100 30 2.1 448 x x 32 29 two- 450 2 460 6 380 470 20 1.37 0.209 314 20 90 30 2.1 456 ∘ x stage x 33 1 double 380 4 450 4 350 500 20 0.72 0.213 252 20 95 30 1.39 311 ∘ x 34 1 double 450 4 380 4 350 500 20 0.87 0.218 254 20 80 30 1.43 329 ∘ x 35 1 double 430 4 510 4 440 500 5 0.43 0.212 249 20 90 30 1.62 308 ∘ x 36 1 single 370 6 — — 360 470 20 0.46 0.214 203 20 100 30 1.47 283 ∘ x 37 1 single 520 1 — — 470 470 20 0.82 0.229 218 18 100 30 1.32 338 ∘ x

While the present invention has been particularly described with reference to specific embodiments thereof, it is obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. This application claims priority to Japanese Patent Application No. 2014-053702 filed Mar. 17, 2014, the entire contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

As described above, the present invention can provide 7xxx-series aluminum alloy sheets for automobile components, where the aluminum alloy sheets have strength, formability, and corrosion resistance all at satisfactory levels, even after natural aging at room temperature. Accordingly, the present invention is advantageously applicable not only to frames, pillars, and other automobile structural components, which contribute to reduction in body weight, but also to structural components for other uses. 

1. An Al—Zn—Mg alloy sheet, comprising: in mass percent, Zn in a content of 3.0% to 6.0%; Mg in a content of 2.5% to 4.5%; Cu in a content of 0.05% to 0.5%; and wherein, the Zn content [Zn] and the Mg content [Mg] satisfies [Zn]≧−0.3[Mg]+4.5; grain-boundary precipitates observed with a transmission electron microscope at 60000-total magnification have an average compositional ratio of Zn to Mg of from 0.5 to 3.0 in a microstructure of the sheet subjected to a natural aging at room temperature subsequent to a solution treatment and quenching; intragranular precipitates observed with a transmission electron microscope at 60000-total magnification have an average compositional ratio of Zn to Mg of from 1.5 to 3.5 in a microstructure of the sheet further subjected to one of a two-stage artificial aging and a single-stage artificial aging after the natural aging at room temperature where the two-stage artificial aging comprises: a first-stage heat treatment at a temperature of from 70° C. to 100° C. for 2 hours or longer; and a second-stage heat treatment at a temperature of from 100° C. to 170° C. for 5 hours or longer, and the single-stage artificial aging comprises a heat treatment at a temperature of from 100° C. to 150° C. for 12 to 36 hours.
 2. The Al—Zn—Mg alloy sheet according to claim 1, further comprising: in mass percent, at least one of (a) and (b) as follows: (a) at least one element selected from the group consisting of: Zr in a content of 0.05% to 0.3%; Mn in a content of 0.1 to 1.5%; Cr in a content of 0.05% to 0.3%; and Sc in a content of 0.05% to 0.3%, and (b) at least one element selected from the group consisting of: Ag in a content of 0.01% to 0.2%; and Sn in a content of 0.001% to 0.1%.
 3. The Al—Zn—Mg alloy sheet according to claim 1, wherein the Zn content [Zn] and the Mg content [Mg] satisfies [Zn]≧−0.5[Mg]+5.75, and the sheet after the artificial aging has a 0.2% yield strength of 400 MPa or more.
 4. The Al—Zn—Mg alloy sheet according to claim 2, wherein the Zn content [Zn] and the Mg content [Mg] satisfies [Zn]≧−0.5[Mg]+5.75, and the sheet after the artificial aging has a 0.2% yield strength of 400 MPa or more. 